1. Field of the Invention
The present invention relates to a tank cover which covers an independent spherical tank loaded for example in a ship, such as a liquefied natural gas carrier (LNG carrier).
This application is based on Japanese Patent Application No. 2003-331478, the content of which is incorporated herein by reference.
2. Description of Related Art
In a ship such as an LNG carrier, as shown in FIG. 5, for example, independent spherical tanks 15 which accommodate liquefied natural gas are loaded so that a plurality of them are arranged along the longitudinal direction of a hull 10 (left and right direction in FIG. 5), partially protruding above an upper deck 11 of the hull 10.
In such a ship, the protruding part of the each of the independent spherical tanks 15 which protrudes above the upper deck 11 is covered by a tank cover 20 for protecting the independent spherical tank 15 from outside air or sea water. The tank cover 20 is a hollow body comprising an approximate hemispherical part 21 and an approximate cylindrical part 22. A bottom end part 23 of the approximate cylindrical part 22 (bottom end part 23 of the tank cover 20) is jointed to the upper deck 11 of the hull 10 by welding.
If a vertical bending moment occurs in the hull 10 due to the load received when the ship is navigating in waves, then as shown in the side view of FIG. 6A, the tank cover 20 having the bottom end part 23 welded to the upper deck 11 of the hull 10 deforms in the opposite direction to the deformation of the hull 10, and therefore a large load is caused in the bottom end part 23 of this tank cover 20 due to compression and tension. At the same time, the approximate cylindrical part 22 receives a deformation by a tensile force in the longitudinal direction of the hull and a deformation by a compressive force in the beam direction (refer to the plan view of FIG. 6B). Particularly, in the bottom end part 23 of the tank cover 20, the load and the forced deformation occurring in the front and back ends (both end parts in the direction along the longitudinal direction of the hull 10, part A in FIG. 5), and the left and right ends (both end parts in the direction along the width direction of the hull 10, part B in FIG. 5) become very large.
For example, when the load compressing toward the upper deck 11 of the hull 10 and the deformation by a compressive force toward the inside (right side in FIG. 7) occur in the bottom end part 23 of the tank cover 20, then as shown in FIG. 7, the bottom end part 23 of the tank cover 20 deforms so as to bend toward the inside of the tank cover 20 (right side in FIG. 7), and as shown on right side in FIG. 7, the total stress of the combined axial stress and bending stress acts upon the bottom end part 23.
Since such stress act on the bottom end part 23 of the tank cover 20, it is very important for the ship to maintain the fatigue strength of the bottom end part 23 of the tank cover 20.
On the other hand, in Japanese Unexamined Patent Application, First Publication No. Hei 1-164696, a technique is disclosed where a large opening is formed in a transverse bulkhead 12 of the hull 10 which separates spaces accommodating the adjacent independent spherical tanks 15 so as to make this transverse bulkhead 12 a flexible structure, whereby the stress acting on the bottom end part 23 of the tank cover 20 is alleviated.
In this technique disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 1-164696, of the front and back ends and the left and right ends of the bottom end part 23 of the tank cover 20 on which particularly large loads act, the stress acting on the front and back ends can be sufficiently alleviated since the front and back ends are arranged just above the transverse bulkhead 12, being a flexible structure.
However, the stress acting on the left and right ends of the bottom end part 23 can not be sufficiently alleviated, since the left and right ends are arranged just above the side plating 13, being a rigid structure. Therefore, in the technique disclosed in Japanese Unexamined Patent Application, First Publication No. Hei 1-164696, it has been difficult to satisfy a high standard for fatigue strength, which has been recently required.